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I have recently built a 80386 system. Lacking any AT PSU I connected a PicoPSU using an ATX->AT power adapter. Only card connected was a gpu (Trident TVGA 9000i). System performed BIOS checkup sequence without errors and reached boot stage. I did not connect a MultiIO card so did not proceed to booting.

PicoPSU is not v.1.x of ATX standard and so does not provide any -5 Volts capability. From what I have read, -5V is needed only by ISA bus and only from those cards requiring it (5th pin of back side of card, named B05 in ISA pinout.

My question is: If I stick to ISA cards which do not need -5v (lack of B05 contact in card) do I face any other risks when resorting to an ATX->AT conversion so as to power the board?

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If it works and the voltages are within tolerance then it should be OK to use it. The only thing that might affect things is that normal ATX supplies may regulate voltages according to 3.3V output, so normal ATX supplies may not work properly of there is no load on 3.3V as AT does not use 3.3V supply. The PicoPSU may work without 3.3V load just fine. And there are adapters to make -5V from -12V if necessary.

  • I think even a small load like a LED would solve the 3.3V problem (just to close the regulation loop of PSU)... anyway -12V to -5V is really just few diodes in serial or one zener diode taking 7V from the 12V ... not even a resistor is needed ... there are 6V9 zener diodes out there ... – Spektre Jun 23 at 9:40
  • That is very dubious to drop voltage with just diodes or zeners. Usually a 7905 linear regulator is used. Or a charge pump to make the negative voltages from positive. – Justme Jun 23 at 10:39
  • I know but one or few diodes can fit directly onto/into a cable (just before the connector with very small effort or change in the HW) and -5V is usually used just for a reference so its very low power anyway ... – Spektre Jun 23 at 12:14
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    It is mainly used only by a few sound cards. And if it is used, it is used as the negative power supply for audio op-amps, not as a reference. – Justme Jun 23 at 14:24
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    Dropping voltage with diodes here is ridiculous. You can get a proper buck converter on Amazon for a few bucks (pun intended), or on aliexpress for a few pennies. – R.. Jun 23 at 22:06

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